Not Applicable.
1. Field of the Invention
The present invention generally relates to the measurement of inductance, and more particularly to inductive vehicle detectors.
2. Description of the Related Art
Inductive wire-loop vehicle detectors of the prior art are typically based on the Colpitts oscillator using a wire-loop sensor. A simple wire-loop has two terminals that are typically connected to the rest of the detector circuit through a pair of lead wires. The lead wires connected to the primary coil of a transformer serving as a common-mode choke typically having 40 dB common-mode noise attenuation. The secondary coil of the transformer is connected to a capacitor effectively forming an inductance-capacitance-resistance (LCR) circuit with the wire-loop.
In the typical Colpitts oscillator-based detector, one leg of the LCR circuit is connected to a positive direct current (DC) power supply terminal. Because of this arrangement, the common-mode noise appearing at the secondary coil of the transformer is converted to differential noise as the common-mode current flowing through the leg of the secondary coil tied to the positive power-supply terminal is drained away. Consequently, the common-mode current flowing through the other leg of the secondary coil charges the capacitor of the LCR circuit. These current flows create a differential noise voltage, which is added to the existing differential noise on the circuit. The largest component of common-mode noise is typically power-line noise around 60 Hz. For a typical two-meter loop having three turns of wire, the differential noise induced by a 60 Hz power line is at 60 Hz and its harmonics. The primary method for canceling ambient noise in prior-art detectors is to integrate the sampling period of the detector over a time chosen to coincide with the local power-line voltage period.
Additionally, where multiple wire-loop sensors are placed in close proximity, crosstalk is a concern. Crosstalk between detectors is a function of the inductive coupling between the wire-loops and the transformers as well as the relative phase and amplitudes of the oscillating signals on the loops. The primary method for mitigating the effects of crosstalk in prior-art detectors is to use different capacitance values in the LCR circuits. This tends to randomize the relative phase of the oscillating signals on adjacent loops over time.
The primary methods for minimizing crosstalk and canceling ambient noise described above tend to limit the sampling rate of prior-art wire-loop sensors to approximately 60 Hz, which is well below what is desirable for recording repeatable inductive signatures on vehicles traveling at highway speeds.
It is desirable to isolate signal from noise in an inductive vehicle detector. Inductance is typically measured indirectly as a function of the resonant frequency of an LCR circuit in which the oscillation frequency is approximately inversely proportional to the square root of the product of inductance and capacitance. In practice, significant errors in the measurement of this oscillation frequency are typical.
In the absence of noise errors, the measured inductance of a wire-loop is independent of the polarity of the excitation current used to make the LCR circuit oscillate. However, when random and non-random differential noise is induced into the circuit, typically through the wire-loop, lead-wire, and transformer, the resulting inductance measurement errors strongly depend on the polarity of the excitation current. By employing an oscillator circuit having two balanced capacitors and by controlling the polarity of the excitation current, the effects of common-mode and differential noise can be greatly reduced with minimal effect on the inductance measured at the wire-loop sensor.